The present invention concerns a liquid crystal display device. A liquid crystal display device in which light modulation is conducted by the change of birefringence value of a liquid crystal layer by applying a voltage thereto, has already been known. In the liquid crystal display device of this type, however, since the distance of the actual path of light in the liquid crystal layer varies depending on a viewing angle and apparent birefringence value varies depending on the relative angle between the aligning direction of liquid crystal molecules and the viewing angle direction, viewing angle dependency of the display characteristic is remarkably great and an angular range capable of visually recognizing the display is extremely narrow.
In order to reduce such a viewing angle dependency of the display characteristic, there has been proposed to dispose a birefringent layer between a liquid crystal layer and a polarizer. For instance, Japanese Patent Laid Open Sho 62-210423 discloses a DAP system-ECB type liquid crystal display device wherein light modulation is conducted by applying a voltage to a liquid crystal layer including the liquid crystal molecules subjected to homeotropic alignment, to change the birefringence value in the liquid crystal layer, and wherein a polymer film prepared so as to have a negative optical anisotropy is disposed as the birefringent layer, and viewing angle compensation is conducted by the birefringent layer.
Further, Japanese Patent Laid Open Sho 64-519 (1989) has proposed a STN type liquid crystal display device in which an optically anisotropic member having an optical property complementary to that of a liquid crystal is disposed between the liquid crystal cell and a polarizer, and thereby viewing angle compensation is conducted. That is, it is substantially required in this device that the optical anisotropic member has the same values for .DELTA. n.d (the product of the refractive index anisotropy and the thickness) and for twist angle (the twisting direction being opposite) as those of the liquid crystal, and that the optical axis on the surface of the anisotropic member opposed to the liquid crystal cell forms an angle of 90.degree. to the rubbing direction on the upper electrode substrate of the liquid crystal cell.
As has been described above, the characteristics required for such a birefringent layer for viewing angle compensation generally depend on the modes of the liquid crystal display device (STN, ECB, DAP, HAN, TB, etc.), cell parameters (direction of the optical axis of the polarizer, birefringence value .DELTA. n.d, etc. of the liquid crystal layer), kinds of the substrate and the like, and may be various factors. A birefringent layer for viewing angle compensation in which the refractive index n.sub.x, n.sub.y within a plane of the birefringent layer is greater than the refractive index n.sub.z in the direction perpendicular to the plane and which is useful for viewing angle compensation of especially DAP, HAN or a skew alignment-type liquid crystal display device, has not been satisfactorily obtained with the reasons as described below.
(1) In a uniaxially stretched polymer as described in Japanese Patent Laid Open Sho 64-519, since n.sub.x is different from n.sub.y, the direction capable of viewing angle compensation is limited to a narrow range.
(2) In a polymer as described in Japanese Patent Laid Open Sho 62-210423, an extremely special molding is necessary, so that preparation of a homogenous film is difficult and control of the birefringence value is also extremely difficult.
(3) In a cell including a usual smectic or nematic liquid crystal comprising rod-like molecules subjected to homogeneous alignment, since the refractive index n in the major axis direction of molecules is generally greater than the refractive index n in the minor axis direction of molecules, the range capable of viewing angle compensation is restricted by the same reason as in (1) above.
(4) Japanese Patent Application Laid Open Sho 64-519 discloses the use of a liquid crystal polymeric film showing a cholesteric phase as the optical isomer but, since the cholesteric pitch of the film becomes greater than the wavelength of visible ray for satisfying the foregoing conditions in view of practical use, it is difficult to render the refractive index within the plane of the birefringent layer isotropic to the visible ray.
The present inventors have made a study on a liquid crystal display device having a birefringent layer for viewing angle compensation free from the foregoing drawbacks and, as a result, have succeeded in remarkably reducing the viewing angle dependency of the display characteristic of the device by using, as a birefringent layer, a liquid crystal or polymer film prepared by a simple method, having a refractive index; n.sub.x =n.sub.y &gt;n.sub.z and showing a homogeneous cholesteric phase, and the present invention has been achieved based on the above finding.
The object of the present invention is to provide a liquid crystal display device having display characteristics with improved viewing angle dependency.